1. Field of the Invention
The present invention relates to an apparatus for detecting the amplitude and phase of an a.c. signal (a.c. voltage or current) that is used for the control or protection of electrical devices, e.g., circuit breaker, power converter or power regulator, connected to a power system.
2. Description of the Prior Art
FIG. 23 shows the arrangement of a conventional amplitude and phase detecting apparatus. Reference numeral 1 denotes an object system for which the amplitude and phase of an a.c. signal are detected, 2 is a single-phase a.c. power source having a voltage amplitude of Vo and angular frequency of .omega..sub.0, 3 is a load connected to the a.c. power source, and 4 is an a.c. voltage detector (a.c. voltage detection means) which detects the a.c. voltage (a.c. signal) of the object system 1 and produces a voltage signal v that is proportional to the a.c. voltage. The apparatus may be designed to detect the a.c. current instead of the a.c. voltage.
Indicated by 5 is an amplitude detector which detects the amplitude V of the a.c. voltage v, and this device is disclosed in Japanese Patent Publication No. 56-28469. Indicated by 6 is a phase detector which detects the phase .theta. of the a.c. voltage v, and this device is disclosed in the Electric Engineering Handbook, p. 447, published in Feb. 1988 by The Institute of Electrical Engineers of Japan.
Indicated by 7 is a rectifying circuit which implements the single-phase full-wave rectification for the voltage signal v provided by the a.c. voltage detector, 8 is a first-order lag circuit for attenuating the even harmonics of the voltage signal v thereby to produce a signal that is proportional to the amplitude of the voltage signal v, 9 is a zero-cross point detection circuit for detecting the zero-cross point of the voltage signal v, and 10 is a phase comparison circuit which detects the phase difference between the detected zero-cross point and the output phase of a voltage-controlled oscillator (VCO) 12 which oscillates so that the phase difference provided through a low-pass filter 11 is minimum at which the output phase represents the phase .theta. of the voltage signal v.
Low pass filter 11 is used to remove higher harmonics included in a phase difference signal detected by phase comparison circuit 10.
In operation, the a.c. voltage detector 4 detects the a.c. voltage of the object system 1 and produces a voltage signal v that is proportional to the a.c. voltage. The voltage signal v is fed to the amplitude detector 5 and phase detector 6, by which the amplitude V and phase .theta. of the a.c. voltage are detected.
In the amplitude detector 5, the rectifying circuit 7 implements the single-phase full-wave rectification for the voltage signal v to produce a signal, which includes even harmonics of the voltage signal v in addition to a d.c. component. The first-order lag circuit 8 filters out the even harmonics, thereby to detect the d.c. component that represents the amplitude V of the voltage signal v.
In the phase detector 6, the zero-cross point detection circuit 9 detects the zero-cross point of the voltage signal v, and the phase comparison circuit 10 detects the phase difference between the zero-cross point and the output phase of the voltage-controlled oscillator 12 which oscillates so that the phase difference is minimum at which the output phase represents the phase .theta. of the voltage signal v. Harmonic components included in the phase difference signal provided by the phase comparison circuit 10 are eliminated by the low-pass filter 11 so that phase .theta. of the a.c. voltage is detected accurately.
This circuit arrangement, however, includes a first-order lag circuit 8 in the amplitude detector 5 and a low-pass filter 11 in the phase detector 6, and these circuits cause a time lag because of their characteristics in detecting the amplitude V and phase .theta. of the a.c. voltage.
Another conventional amplitude and phase detection apparatus disclosed in Japanese Patent Laid-open No. 1-301183 uses an observer to split a detected a.c. signal into two a.c. signals that are out of phase with each other by 90.degree. and calculates the amplitude of the a.c. signal based on the split a.c. signals. However, the observer generally has a delay of response, causing a phase lag in its output, and therefore this apparatus either cannot detect the amplitude V of an a.c. voltage instantaneously.
The conventional a.c. signal amplitude and phase detection apparatus arranged as described above is not capable of detecting the amplitude and phase of an a.c. signal instantaneously due to the use of the first-order lag circuit 8 and low-pass filter 11 that cause a phase lag because of their phase characteristics, and consequently it cannot perform perfectly the control or protection of electrical devices in the system.